here's another idea I just had, tho it's less immediately doable. I'm sure some of you have heard about Dodgeball? Basically, when you go out, you send a message to them saying where you are, and they send a message to your friends telling them. And possibly to their friends, if there's someone nearby. It wouldn't be hard to set up something like this for cons. You just send a message indicating which room you're in (or going to), so everybody can keep track, and find people if they want.
Something I might take a poke at for the next con.